Method for manufacturing hollow core fiber and method for manufacturing preform for hollow core fiber

Abstract

A method for manufacturing an antiresonant hollow-core fiber having a hollow core extending along the longitudinal axis of the fiber and an internal cladding region including a plurality of antiresonant elements and surrounding the hollow core is known. The known method includes the steps of providing a cladding tube having an internal bore and a longitudinal axis, a cladding wall defined by an inner and outer side extending along the longitudinal axis of the cladding tube, providing a plurality of antiresonant element preforms, placing the antiresonant element preforms at targeted positions inside the cladding wall to form primary preforms for the hollow-core fiber having a hollow core region and a cladding region, and drawing the primary preforms to form the hollow-core fiber or further processing the primary preforms to form secondary preforms. Starting from this, in order to achieve high precision and accurate positioning of the antiresonant elements in a sufficiently stable and reproducible manner, it is proposed that providing the cladding tube includes the processing step of providing a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube by cutting in the targeted region of the cladding wall.

Description

[Technical field]

[0001] The present invention relates to a method of making an anti-resonant hollow-core fiber having a hollow core extending along a longitudinal axis of the fiber and a cladding region including a plurality of anti-resonant elements and surrounding the hollow core, the method comprising: (a) providing a cladding tube having an internal cladding bore and a cladding longitudinal axis, the cladding tube wall being defined by an interior and an exterior and extending along the cladding longitudinal axis; (b) providing a plurality of anti-resonant element preforms; (c) placing the anti-resonant element preform at a target location inside the cladding wall to form a primary preform for a hollow-core fiber having a hollow core region and a cladding region; (d) drawing the primary preform into a hollow-core fiber or further processing the primary preform into a secondary preform from which a hollow-core fiber is drawn; Equipped with In the further processing step, (i) Stretching; (ii) Collapse, (iii) collapsing and simultaneous stretching; (iv) Collapse of additional cladding material; (v) collapse of additional cladding material followed by stretching; (vi) collapsing and simultaneously stretching additional cladding material; One or more of the thermoforming processes are carried out once or repeatedly.

[0002] The present invention further relates to a method for making an anti-resonant hollow-core fiber preform having a hollow core extending along a longitudinal axis of the fiber and a cladding region surrounding the hollow core, the cladding region including a plurality of anti-resonant elements, the method comprising: (a) providing a cladding tube having an internal cladding bore and a cladding longitudinal axis, the cladding tube wall being defined by an interior and an exterior and extending along the cladding longitudinal axis; (b) providing a multiple anti-resonant element preform; (c) placing the anti-resonant element preform at a target location inside the cladding wall to form a primary preform for a hollow-core fiber having a hollow core region and a cladding region; (d) optionally further processing the primary preform into a secondary preform for a hollow-core fiber; Equipped with In the further processing step, (i) Stretching; (ii) Collapse, (iii) collapsing and simultaneous stretching; (iv) Collapse of additional cladding material; (v) collapse of additional cladding material followed by stretching; (vi) collapsing and simultaneously stretching additional cladding material; One or more of the thermoforming processes are carried out once or repeatedly. [Background technology]

[0003] Conventional single-mode optical fibers made from solid materials have a glass core surrounded by a cladding region made of glass with a low refractive index. The propagation of light is based on total internal reflection between the core and cladding regions. However, the interaction of the guided light with the solid material is associated with increased delay times in data transmission and a relatively low damage threshold for energetic radiation.

[0004] These disadvantages are avoided or mitigated by "hollow-core fibers", whose core consists of a vacuum cavity filled with a gas or liquid. In hollow-core fibers, the interaction of light with the glass is reduced compared to solid-core fibers. Since the refractive index of the core is smaller than that of the cladding, light propagation by total internal reflection is not possible, and light is usually considered to leak from the core into the cladding. Depending on the physical mechanism of light propagation, hollow-core fibers are distinguished into "photonic bandgap fibers" and "antiresonant reflecting fibers".

[0005] In a "photonic bandgap fiber", a hollow core region is surrounded by a cladding that contains a periodic arrangement of small hollow channels. The periodic structure of the hollow channels in the cladding has an effect called "photonic bandgap" in semiconductor technology, which means that light in a certain wavelength range that scatters into the cladding structure cannot spread laterally within the cladding because it interferes constructively in the central cavity based on Bragg reflection.

[0006] In hollow-core fiber embodiments, called "antiresonant hollow-core fibers" (ARHCF), a hollow core region is surrounded by an inner cladding region and is located within so-called "antiresonant elements" (or "antiresonant elements"; abbreviation "AREs"). The walls of the antiresonant elements, evenly distributed around the hollow core, function as an antiresonantly operated Fabry-Perot cavity that can reflect incoming light and pass it through to the fiber core.

[0007] This fiber technology allows for reduced optical attenuation, a very wide transmission spectrum (even in the ultraviolet or infrared wavelength ranges), and low latency during data transmission.

[0008] Potential applications of hollow-core fibers lie in the fields of high-performance beam control for data transmission, material processing, and modal filtering, and in nonlinear optics, particularly for generating supercontinuums from the extreme ultraviolet to the infrared wavelength range.

[0009] Prior Art A drawback of anti-resonant hollow-core fibers is that higher order modes are not automatically suppressed, so they often do not become purely single mode over long transmission distances, resulting in poor quality output beams.

[0010] Francesco Poletti, "Nested antiresonant nodeless hollow core fiber"; Optics Express, Vol. 22, No. 20 (2014); DOI: 10.1364 / OE 22.023807, proposes a fiber design in which the antiresonant element is not formed as a simple single structural element, but is composed of multiple structural elements nested within each other. The nested antiresonant elements are designed in such a way that the higher-order core modes are phase-matched to the cladding modes and suppressed, but the fundamental core mode is not suppressed. This ensures that the fundamental core mode always propagates, making the hollow core fiber effectively single-mode over a limited wavelength band.

[0011] Efficient mode suppression depends on the central wavelength of the propagating light as well as on structural parameters of the fiber design, such as the radius of the hollow core and the diameter difference of the nested ring structures within the anti-resonant element.

[0012] From EP 3136143 A1 an anti-resonant hollow-core fiber (called "hollow-core fiber without band gap") is known, whose core is capable of propagating other modes besides the fundamental mode. For this purpose, the core is surrounded by an inner cladding with "anti-resonant elements" that provide phase matching of the anti-resonant mode and higher modes. The production of hollow-core fibers is carried out by the so-called "stack and draw" method, in which a preform is formed by arranging and fixing the starting elements in an axially parallel collection, and then drawing the preform. Here, a cladding tube with a hexagonal inner cross section is used, to whose inner edge six so-called "ARE preforms" (anti-resonant element preforms) are fixed. The hollow-core fiber is formed by drawing the preform in two stages.

[0013] From International Patent Application 2018 / 169487 A1 a method for manufacturing a preform for an antiresonant hollow core fiber is known, in which a first cladding region is made up of a number of rods and a second cladding region is made up of a number of tubes surrounded by an outer cladding tube. The rods, tubes and cladding tube are joined by a "stack and draw" method to form the preform. Before the preform is drawn, sealing is performed by applying a sealant to the preform ends. As sealant a UV adhesive is used, for example. Summary of the Invention [Problem to be solved by the invention]

[0014] Anti-resonant hollow-core fibers, and especially those with nested structural elements, have complex internal geometries that make them difficult to manufacture accurately and reproducibly. This is compounded by the fact that resonance or anti-resonance conditions cannot be met without small dimensional tolerances on the magnitude of the operating wavelength of the propagating light. Deviations from the target shape can be caused during the construction of the fiber preform, but can also occur during the fiber drawing process due to inappropriate deformations that are not to scale.

[0015] In the known "stack and draw" method, a large number of elements must be joined in precise positions. For example, to manufacture a hollow-core fiber of the "NANF" design known from the literature mentioned at the beginning, six anti-resonant element preforms, each consisting of an anti-resonant element outer tube (abbreviated as "ARE outer tube") and an anti-resonant element inner tube (abbreviated as "ARE inner tube") welded to one side of the inner cladding surface of the outer ARE tube, must be attached inside the cladding tube.

[0016] In order to achieve low attenuation values ​​and wide propagation ranges, the azimuthal position of the anti-resonant element inside the cladding tube is also important, in addition to the uniform wall thickness of the anti-resonant element walls, which cannot be easily achieved by the "stack and draw" method. It is an object of the present invention to provide a manufacturing method for achieving anti-resonant hollow-core fibers at low cost, avoiding the limitations of conventional manufacturing methods.

[0017] In particular, it is an object of the present invention to provide a method for manufacturing anti-resonant hollow-core fibers and anti-resonant hollow-core fiber preforms, which makes it possible to achieve high precision of the structural elements and exact positioning of the anti-resonant elements within the fiber in a sufficiently stable and reproducible manner.

[0018] Furthermore, the drawbacks of the conventional "stack and draw" method, in which the required construction precision, in particular the uniform wall thickness and precise positioning of the anti-resonant elements in defined azimuthal positions, are not easily achievable, must be avoided as far as possible. [Means for solving the problem]

[0019] With regard to a method for manufacturing an antiresonant hollow-core fiber, this problem is solved according to the invention, starting from a method of the type mentioned at the beginning, in that the step of providing a cladding tube according to step (a) comprises the process measure of providing a longitudinal structure extending in the direction of the longitudinal axis of the cladding tube by cutting in the region of the target position of the cladding tube wall.

[0020] The starting point for the manufacture of anti-resonant hollow-core fibers is a preform, referred to herein as the "primary preform." This preform includes a cladding tube, which contains a precursor or preform for forming an anti-resonant element in the hollow-core fiber (referred to herein for short as the "anti-resonant element"). Although the primary preform can be drawn to form a hollow-core fiber, it is usually further processed to produce a preform, referred to herein as the "secondary preform." If necessary, the hollow-core fiber is produced by drawing this secondary preform. Alternatively, the primary or secondary preform is surrounded by one or more outer cylinders, forming a coaxial collection of components, which is directly drawn to form the hollow-core fiber. In this case, the general term "preform" is to be understood as the name of the component or coaxial collection of components from which the hollow-core fiber is finally drawn.

[0021] The positioning accuracy of the preform is improved by previously structuring the cladding tube by cutting machining.

[0022] By cutting is meant cutting machining techniques such as turning, cutting, drilling, sawing, milling or grinding, which provide more precise and extremely fine structures compared to other known deformation techniques using heat and pressure and avoid surface contamination by forming tools such as nozzles, presses or casting moulds. Preferably, the longitudinal structure of the cladding inner wall is produced by drilling, sawing, milling, cutting or grinding.

[0023] This process results in a longitudinal structure extending in the direction of the cladding tube longitudinal axis, which serves as a positioning aid for the anti-resonant element preform, facilitating its placement in a defined position inside the cladding tube, the longitudinal structure being accessible at least from the inside of the cladding tube and may extend through the entire cladding tube wall to the outside.

[0024] The present invention allows for precise and reproducible manufacture of anti-resonant hollow-core fibers, as well as preforms therefor.

[0025] A cladding tube is provided which preferably has a circular inner cross section, the longitudinal structures being configured as longitudinal grooves or as longitudinal slots inside the cladding tube wall.

[0026] For the purpose of structuring the inside of the cladding tube, a cladding tube with a circular inner cross section is provided, so that the manufacturing costs of the cladding tube itself are reduced compared to a cladding tube with a hexagonal socket-shaped internal bore.

[0027] The positioning points can have different shapes. Longitudinal structures configured as longitudinal grooves on the inside of the cladding wall or as radially continuous longitudinal slots can be produced simply and precisely, for example by milling, drilling or cutting. The internal shape of the grooves and slots can be, for example, semicircular, semi-elliptical, V-shaped, U-shaped or polygonal. It can also be a depression or recess tapering from the inside to the outside.

[0028] It is advantageous, particularly in the case of continuous longitudinal slots, if the cladding tube has an end edge and the longitudinal structure terminates before this end edge.

[0029] Depending on the shape of the longitudinal structure and the shape and size of the anti-resonant element preform, multiple contact points or a large contact surface between the anti-resonant element preform and the longitudinal structure may result during fixing. In a particularly proven method, the anti-resonant element preforms contact the longitudinal structure at two edges each when the anti-resonant element preforms are placed at their target positions according to step (c).

[0030] This provides more stability than a single contact line and ensures that the anti-resonant element preform does not "move around" during stretching. The external dimensions (i.e., outer diameter) of the preform are determined by the maximum width S of the longitudinal structure. B, so that the preform rests on the longitudinal groove or slot and does not sink completely in. Below the contact edge, a cavity can be provided in the longitudinal groove or slot into which gas can be pumped or sucked out, if necessary.

[0031] In another preferred method, the anti-resonant element preform has a maximum longitudinal structure width S B Here, the grooves can be made shallow, for example, to allow the grooves to set only preferred positions relative to the anti-resonant element preform.

[0032] In a further preferred method, the anti-resonant element preform is also formed to have a maximum width S of the longitudinal structure. B 1, but where the anti-resonant element preform is at least partially surrounded by the longitudinal structure, the anti-resonant element preform is completely fixed to the inner wall of the cladding tube.

[0033] In a particularly advantageous embodiment of the method, an insertion tube is additionally inserted into the cladding tube inner bore, the anti-resonant element preforms being sealed in pressure chambers between the longitudinal structure and the insertion tube, respectively.

[0034] In this way, a hollow channel is respectively formed in the longitudinal structure of the preform, into which the anti-resonant element preform is inserted and which is defined by the longitudinal structure and the wall of the insertion tube.

[0035] When the process is carried out according to step (d), the insertion tube is deformed, for example during stretching or collapsing, by forcing gas into the hollow channel and applying an internal pressure to the pressure chamber, whereupon the wall portion of the insertion tube in contact with the longitudinal structure (precisely the pressure chamber) is deformed and forms an elongated bulge on the inside of the insertion tube, which protrudes inwards towards the hollow core and serves as an anti-resonant element.

[0036] In a variant of the proven method, the anti-resonant element preforms each have at least one anti-resonant element and at least one capillary tube connected to this anti-resonant element, the capillary tube being accommodated in a recess of the longitudinal structure and being deformed by applying an internal pressure to the capillary tube inner bore during stretching of the preform and / or semi-finished product.

[0037] In this case, the capillary tube is used on the one hand to fix the anti-resonant element preform to the longitudinal structure, which for this purpose is provided with a recess with a sufficient dimension to accommodate the capillary tube, and on the other hand, the capillary tube is subjected to an additional pressure during the elongation process, so that the capillary tube positions the original anti-resonant element of the preform as a result of its deformation and expansion, the deformed capillary tube walls then being able to form the additional anti-resonant element of the preform.

[0038] In a variation of the preferred method, the longitudinal structure has longitudinal slots distributed around the circumference of the cladding wall, and the anti-resonant element preforms are disposed in the longitudinal slots.

[0039] The longitudinal slots penetrate the cladding wall from the inside to the outside, except in both end face regions of the cladding. The longitudinal slots have parallel longitudinal edges and a maximum slot width S B with the anti-resonant element preform preferably connected to the longitudinal edges.

[0040] In this case, the anti-resonant element preform has a maximum width S B Since the preform has a width dimension greater than the longitudinal slot, it will not sink completely into the longitudinal slot.

[0041] To ensure a more precise geometry, the bond between the anti-resonant element preform and the respective longitudinal edge is softened and simultaneously stretched, by elongating the assembly consisting of the cladding tube and the anti-resonant element preform, the latter being connected to the cut edge inside the cladding tube over its entire length.

[0042] The lower region of the longitudinal edge is a cavity into which gas can be pumped or sucked out.

[0043] In particular, with regard to the low optical attenuation and wide optical transmission bandwidth of hollow-core fibers, it has proven particularly advantageous if the anti-resonant element preforms are arranged in odd symmetry around the hollow core.

[0044] In a preferred method variant, the placement of the antiresonant element preform and / or the drawing of the primary preform and / or the drawing of the hollow core fiber are performed using amorphous SiO 2 This includes fastening and / or sealing measures using particle-containing sealants or bonding materials.

[0045] The sealing or bonding material used for sealing or fixing is, for example, amorphous SiO 2 The material is applied between the surfaces to be joined or sealed and is usually in the form of a paste when used. Drying at low temperatures removes the dispersion partially or completely and the material hardens. The sealing or joining material and especially the hardened SiO 2 The containing sealant or bonding material meets the requirements for fixation and compression. The temperature required for drying is 300°C or less, which helps maintain the dimensional stability of the preform and avoids adverse thermal effects. Heating to high temperatures, around 800°C, for example when drawing the preform into a hollow-core fiber, results in further thermal solidification of the sealant or bonding material, making it suitable for forming frosted or clear glass. This occurs by sintering or vitrification, where sintering to frosted glass requires relatively lower temperatures and / or shorter heating times than vitrifying to full transparency. Thus, the sealant or bonding material can be fully compressed by heating and vitrified by heating in a thermoforming process.

[0046] During the thermoforming process, the sealing or bonding material will not decompose and will release almost no impurities, therefore it is characterized by temperature stability and purity during the thermoforming process, and avoids deformation due to different thermal expansion coefficients.

[0047] Sealing and bonding materials may also be advantageously used to seal the open ends of the antiresonant element preform and / or the annular gaps between the individual structural and / or tubular elements of the antiresonant element preform during drawing of the primary preform and / or drawing of the hollow-core fiber.

[0048] In this manner, individual components of the primary preform and / or secondary preform may be subjected to different internal pressures during the drawing or fiber drawing process.

[0049] In a preferred method, tubular structural elements are provided, at least some of which have a wall thickness in the range of 0.2-2 mm, preferably in the range of 0.25-1 mm, thereby further improving the positioning accuracy of the preform in the cladding tube. The cladding tube is then provided with an outer diameter in the range of 90-250 mm, preferably in the range of 120-200 mm. Each of these components has a length of at least 1 m, which is a relatively voluminous structural element for forming an anti-resonant element. This allows for easy handling. Furthermore, the parallelism and vertical alignment of the longitudinal axes of the structural elements are supported by gravity, if the cladding tube and the structural elements are positioned and fixed in the target position, for example at their respective upper end faces, preferably using the above-mentioned sealing or bonding material.

[0050] The above-mentioned technical problem with regard to the production of a hollow-core fiber preform is solved according to the invention, starting from a method of the type mentioned at the beginning, in that the provision of the cladding tube in step (a) comprises the process measure of attaching longitudinal structures extending in the direction of the cladding tube longitudinal axis by cutting in the region of the target position on the cladding tube wall.

[0051] The preform is the starting point for the production of antiresonant hollow-core fiber: an antiresonant hollow-core fiber can be drawn by directly drawing the primary preform, or the primary preform can first be further processed to produce another semi-finished product (also called a "secondary preform") from which the antiresonant hollow-core fiber can be drawn.

[0052] In either case, the manufacture of the preform includes the attachment and connection of the antiresonant element preform and the cladding tube. The positioning accuracy of the preform is improved by prestructuring the cladding tube. The steps for manufacturing the preform are described in detail above in connection with the manufacture of the hollow-core fiber, and those descriptions are incorporated herein by reference.

[0053] definition The following are supplementary definitions for the individual steps and terms in the above-mentioned specification. These definitions are part of the present specification. If there is a substantial discrepancy between any of the following definitions and the rest of the specification, the definitions mentioned in the rest of the specification shall prevail.

[0054] anti-resonance element The anti-resonant element can be a simple structural element or a nested structural element of the hollow-core fiber. It has at least two walls that have negative curvature (convex) or no curvature (flat, straight) when viewed in the direction of the hollow core. Usually, the anti-resonant element is made of a material transparent to the operating light, such as glass (especially doped SiO 2 or undoped SiO 2 ), plastics (especially polymers), composite materials or crystalline materials.

[0055] Anti-resonant element preform / anti-resonant element pre-stage The anti-resonant element preform is a component or a preform component that becomes an anti-resonant element in a hollow-core fiber mainly by simple drawing in a fiber drawing process. The anti-resonant element pre-stage is a component or a preform component that becomes an anti-resonant element preform or directly an anti-resonant element only by deformation. The anti-resonant element preform can be a simple component or a nested component to which additional positioning aids can be fixed. The anti-resonant element preform is originally present in the primary preform.

[0056] The nested anti-resonant element preform forms an anti-resonant element nested within a hollow-core fiber. It is composed of an outer tube and at least one further structural element disposed within the inner bore of the outer tube. The further structural element may be a further tube that is in contact with the inner cladding surface of the outer tube. The outer tube is also referred to as the "anti-resonant element outer tube" or "ARE outer tube" for short, and the further tube is also referred to as the "anti-resonant element inner tube" or "ARE inner tube" for short, or "nested ARE inner tube".

[0057] In the case where the anti-resonant element preforms are nested multiple times within the inner bore of the nested inner ARE tube, at least one further structural element may be disposed, for example a third tube, which is in contact with the inner cladding surface of the nested inner ARE tube.

[0058] When the anti-resonant element preforms are nested multiple times, a distinction is made between a "nested outer ARE inner tube" and a "nested inner ARE inner tube" as necessary to distinguish between the multiple tubes disposed within the outer ARE tube.

[0059] The term "cross section" in connection with cylindrical anti-resonant element preforms and their cylindrical structural elements always refers to a cross section perpendicular to the respective cylinder longitudinal axis and, unless otherwise specified, refers to a cross section of the external contour of the tubular component (and not a cross section of the internal contour).

[0060] Further processing of the primary preform, in particular by a thermoforming process, can produce intermediate products in which the original anti-resonant element preform is present in a modified shape relative to its initial shape, which modified shape is also referred to herein as the anti-resonant element preform or anti-resonant element pre-stage.

[0061] Preform / Primary preform / Secondary preform / Core preform (Cane) A preform is a component from which an anti-resonant hollow-core fiber is drawn. It can be a primary preform or a secondary preform produced by further processing of a primary preform. A primary preform can be an assembly of at least one cladding tube and a preform or precursor for an anti-resonant element housed therein in a loosely or rigidly fixed manner. Further processing of a primary preform into a secondary preform from which a hollow-core fiber is drawn can be performed by: (i) Stretching; (ii) Collapse, (iii) collapsing and simultaneous stretching; (iv) Collapse of additional cladding material; (v) collapse of additional cladding material followed by stretching; (vi) collapsing and simultaneously stretching additional cladding material; The thermoforming process may include performing one or more of the following steps once or repeatedly:

[0062] In the literature, a core preform (English: cane) is a preform obtained by collapsing and / or stretching a primary preform. Usually, the core preform is covered with additional cladding material before or during the drawing of the hollow-core fiber.

[0063] Stretching / Collapse In drawing, the primary preform is stretched to an elongation. This drawing may be performed without simultaneous collapse. The drawing may be performed to scale, so that, for example, the shape and arrangement of the components of the primary preform are reflected in the final drawn product. However, in drawing, the primary preform may not be drawn to size, and the geometry may change.

[0064] Collapsing involves narrowing the internal bore or closing or narrowing the annular gap between the tubular components, and is usually accomplished in parallel with stretching.

[0065] Hollow core / inner cladding region / outer cladding region The assembly consisting of at least one cladding tube and the preform or precursor of the anti-resonant element housed therein in a loosely or rigidly fixed manner is also referred to herein as the "primary preform". This primary preform consists of a hollow core and a cladding region. This cladding region is also referred to as the "internal cladding region" when there is an "external cladding region" formed, for example by collapsing into the assembly, and it is necessary to distinguish between these cladding regions. The names "internal cladding region" and "external cladding region" are also used for the corresponding regions of the hollow-core fiber or intermediate product obtained by further processing of the primary preform.

[0066] The term "inside the tube" is also used as a synonym for "inner cladding surface of the tube," and the term "outside the tube" is also used as a synonym for "outer cladding surface of the tube." The term "internal bore" in reference to a tube does not imply that the internal bore is formed by a drilling operation.

[0067] Cutting processing A mechanical manufacturing method for isolating a workpiece, which means in particular turning, cutting, drilling, sawing, milling or grinding, which results in a longitudinal structure extending in the direction of the cladding tube longitudinal axis, which serves as a positioning aid for the anti-resonant element preform, which is accessible from the inside of the cladding tube and may extend through the entire cladding tube wall to the outside.

[0068] Particle size and particle size distribution SiO 2 The particle size and particle size distribution are 50 This value is characterized based on the SiO 2 It is read from the particle size distribution curve, which shows the cumulative amount of particles according to particle size. 10 Value, D 50 Value, D 90 It is often characterized based on the value of D 10 The value is SiO 2 D indicates a particle size that does not reach 10% of the cumulative amount of particles. 50 Value and D 90 SiO 2 This refers to the particle size at which 50% or 90% of the cumulative amount of particles is not reached. The particle size distribution is detected by scattered light and laser diffraction spectroscopy in accordance with ISO13320.

[0069] Working Example The invention is explained in more detail below on the basis of an embodiment with the aid of the figures, the details of which are shown in the figures. [Brief description of the drawings]

[0070] [Figure 1] FIG. 13 is a diagram of a cladding tube for use in fabricating a hollow-core fiber preform, according to embodiments, that includes longitudinal structures for positioning an anti-resonant element preform. [Diagram 2] FIG. 13 is a diagram of a cladding tube for use in fabricating a hollow-core fiber preform, according to embodiments, that includes longitudinal structures for positioning an anti-resonant element preform. [Diagram 3]FIG. 13 is a diagram of a cladding tube for use in fabricating a hollow-core fiber preform, according to embodiments, that includes longitudinal structures for positioning an anti-resonant element preform. [Figure 4] FIG. 2 is a diagram of a cladding tube with a longitudinal structure and an anti-resonant element preform positioned therein based on contact points and contact surfaces. [Diagram 5] FIG. 2 is a diagram of a cladding tube with a longitudinal structure and an anti-resonant element preform positioned therein based on contact points and contact surfaces. [Figure 6] 13 is a diagram of a further embodiment of a cladding tube comprising a longitudinal structure and an anti-resonating element preform positioned therein. [Figure 7] 13 is a diagram of a further embodiment of a cladding tube comprising a longitudinal structure and an anti-resonating element preform positioned therein. [Figure 8] FIG. 1 illustrates an embodiment of a cladding tube with a longitudinal structure and an anti-resonant element preform positioned therein, with a capillary tube provided for adjustable positioning. [Figure 9] FIG. 1 illustrates an embodiment of a cladding tube with a longitudinal structure and an anti-resonant element preform positioned therein, with a capillary tube provided for adjustable positioning. [Figure 10] FIG. 1 is a process diagram for manufacturing a hollow-core fiber preform using a slotted cladding tube. [Figure 11] FIG. 10b is an enlarged view of a portion of the longitudinal structure and anti-resonant element of FIG. 10c. [Figure 12] 12 is a view of a portion corresponding to FIG. 11, according to another embodiment of the anti-resonant element. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0071] In the production of hollow-core fibers or preforms for hollow-core fibers, a large number of components have to be connected to one another. Furthermore, when carrying out a thermoforming process, it is advantageous to seal gaps and channels present in the preform. As is known from DE 10 2004 054 392 A1, bonding or sealing can be carried out using SiO 2 A D-based sealing or bonding material is used. 50 Value is about 5 μm and D 90 Amorphous SiO with a particle size distribution characterized by a value of about 23 μm 2 A particulate aqueous slurry is produced. Amorphous SiO with a medium particle size of about 5 μm is added to this base slurry. 2 The particles are mixed. The slurry used as the bonding material has a solid content of 90% and is at least 99.9% by weight of SiO 2 It is composed of:

[0072] 1 to 8 are top views of the cladding tube end faces. Each cladding tube 1 has longitudinal grooves 3 on the inside of the cladding tube wall 2. Six longitudinal grooves 3 are symmetrically and evenly distributed around the inner circumference of each cladding tube 1.

[0073] The cladding tube 1 is made of silica glass and has a length of 500 mm, an outer diameter of 73 mm, and an inner diameter of 24 mm.

[0074] Figure 1 shows a deep, narrow longitudinal groove 3 made by drilling, with a nearly closed circular cross section, with a maximum depth of 3 mm and an internal diameter of 4 mm.

[0075] FIG. 2 shows a flat, narrow longitudinal groove 3 of semicircular cross section produced by milling.

[0076] FIG. 3 shows a flat wide longitudinal groove 3, also produced by milling, which has a dished cross section.

[0077] The longitudinal grooves 3 are used as positioning aids for the anti-resonant element preforms 5. These may be a collection of nested elements consisting of an outer ARE tube 5a and an inner ARE tube 5b, as shown in Figures 4 and 5.

[0078] Figure 4 shows deep narrow longitudinal grooves 3 made by drilling, with a nearly closed circular cross section, in which anti-resonant element preforms 5 are bounded at two edges 3a, 3b each. The outer ARE tubes each have an outer diameter of 73 mm. The wall thickness of the inner and outer ARE tubes is approximately the same, 0.35 mm.

[0079] Figure 5 shows flat, wide longitudinal grooves 3, approximately dish-shaped in cross section, made by milling, in which the anti-resonant element preforms 5 rest, each with a wide contact surface 3c. The outer ARE tubes each have an outer diameter of 73 mm. The wall thickness of the inner and outer ARE tubes is approximately the same, 0.35 mm.

[0080] 6 shows a deep and narrow longitudinal groove 3 made by drilling, with a nearly closed circular cross section, in which is housed an anti-resonant element preform 5. The anti-resonant element preform 5 is in this case formed as a simple capillary tube.

[0081] Figure 7 shows a deep and narrow longitudinal groove 3 made by drilling, with a nearly closed circular cross section, in which is housed an anti-resonant element preform 5, which in this case is formed as a simple capillary tube. An insertion tube 8 is inserted into the cladding tube inner bore 7, coaxially with the cladding tube longitudinal axis.

[0082] The outer diameter of the insertion tube 8 is similar to the inner diameter of the cladding tube 1 (see FIG. 1) and its wall thickness is similar to the wall thickness of the anti-resonant element preform. Before the fiber drawing process, the cladding tube 1 is crushed onto the insertion tube 8, thus closing the longitudinal grooves 3. The closed longitudinal grooves form hollow channels which subsequently expand (e.g. during drawing of the preform). This results in the insertion tube 8 having protrusions with convex surfaces facing inwards towards the inner bore 7, which act as additional anti-resonant boundary layers.

[0083] In a method variant not shown in the figures, longitudinal grooves with a substantially closed circular cross section are produced by drilling. An insertion tube is inserted into the cladding tube inner bore coaxially with the cladding tube longitudinal axis. The outer diameter of the insertion tube 8 is similar to the inner diameter of the cladding tube (see FIG. 1) and its wall thickness is similar to the wall thickness of the anti-resonant element preform. Before the fiber drawing process, the cladding tube is crushed onto the insertion tube, thus closing the longitudinal grooves. The closed longitudinal grooves become hollow channels, which subsequently expand (e.g. during drawing of the preform). This results in the insertion tube having protrusions with convex surfaces facing inwards towards the inner bore, which act as additional anti-resonant elements.

[0084] In a variant of the method shown in FIG. 8, the antiresonant element preform 5 is provided with positioning capillaries 9 in addition to the antiresonant elements. The positioning capillaries 9 are respectively inserted into the longitudinal grooves 3, which are made by drilling and have a substantially closed circular cross section. The positioning capillaries 9 can be expanded when the assembly is softened, for example when the preform is stretched, so that the antiresonant elements fixed therein can be brought closer to the center (core region). In this case, the amount of movement can be simply set and controlled by the internal pressure of the hollow channel (positioning capillary 9). This allows, apart from the precise adjustment of the azimuthal position, also the radial position of the antiresonant elements and therefore the core diameter of the antiresonant hollow-core fiber to be adjusted.

[0085] FIG. 9 shows two different embodiments of the anti-resonant element preform 5, each provided with a positioning capillary 9.

[0086] 10-12 illustrate the steps for producing a hollow-core fiber, in which a silica glass cladding tube 1 with longitudinal grooves 3 is used for precise positioning of the antiresonant element preforms. As shown in FIG. 10(a), the wall of the cladding tube 1 is cut longitudinally at predefined azimuthal positions and at equal intervals, e.g. by mechanical sawing, water jet cutting, laser, etc. The number of longitudinal cuts 3 corresponds to the number of antiresonant element preforms 5 to be positioned, six preforms 5 in this example. The longitudinal cuts 3 terminate short of the cladding tube end, so that the end region 12 on the end face side remains closed around and connects the remaining web 14. The cut edges are then vitrified.

[0087] The cut width of the longitudinal slots 3 is uniform and is 2 mm. The anti-resonant element preform 5 has a substantially circular outer cross section with a diameter of 7.4 mm.

[0088] From the top view of the cross section of the intermediate tube 10 along the section line AA in FIG. 10(b), it can be seen that the six longitudinal slots 3 are evenly distributed around the circumference of the tube wall 2, and in this view the anti-resonant element preforms 5 are shown contacting the longitudinal grooves 3 with two contact lines, i.e. cut edges 3a; 3b respectively, and projecting into the cladding tube inner bore 16. For fixing, both ends of the anti-resonant element preforms 5 are glued or welded, for example, to the inside of the cladding tube 1. Alternatively and preferably, the anti-resonant element preforms 5 are glued or welded to the inside of the cladding tube 1 as described above. 2 The anti-resonant element preform is connected to the inner cladding surface of the cladding tube by means of the included sealing and bonding material, local fastening being sufficient for this, in particular in the end regions on both end faces.

[0089] By subsequent stretching of this assembly, the anti-resonant element preforms 5 are connected over their entire length to the cut edges 3a; 3b inside the cladding tube 1. By applying pressure to the inner bore of the cladding tube 1, it can be checked whether the cut seam 3 is completely filled by the anti-resonant element preforms 5. The longitudinal slots 3 are thus used as precise positioning aids, allowing precise positioning and fixing of each anti-resonant element preform 5.

[0090] 10(c), the assembly 17 consisting of the covering tube 1 and the antiresonant element preform positioned therein is covered by a capping tube 15 so that additional cladding material can be added and the core to cladding diameter ratio defined for the hollow-core fiber can be adjusted in the secondary preform 18. In the resulting secondary preform 18, an annular gap 19 is left between the capping tube 15 and the assembly 17.

[0091] As the preform 18 is drawn into a hollow-core fiber, a gas can be pumped into or removed from the hollow channel formed at the cut seam 3 between the cap tube 15 and the fused antiresonant element preform 5 through annular gap 19 to create positive or negative pressure within the hollow channel.

[0092] Thus, if necessary or desired, the radial position of the anti-resonant element preform 5 can be changed and modified within the cladding tube inner bore 16, as shown in Fig. 11, where sketch (a) shows the anti-resonant element preform 5 with the outer ARE tube 5a and the nested inner element (inner ARE tube 5b) in an initial position, and sketch (b) shows the outer ARE tube wall partially indented by pressure and heat, and the inner ARE tube 5b fixed in the indentation, with its radial position changed with respect to its initial position. In this way, fiber designs with non-hexagonal symmetry and especially non-integer symmetry can also be realized, unlike the conventional "stack and draw" method.

[0093] It is also possible to "fold" the wall sections of the anti-resonant element preform 5 inside the anti-resonant element preform by applying pressure inside the hollow channel. In Fig. 12(a) the anti-resonant element preform 5 with a simple outer ARE tube 5a (without an additional nested inner ARE tube) is shown in an initial position. Under the action of heat and pressure the wall sections between both contact lines expand inwards. As shown in Fig. 12b, inside the outer ARE tube 5a another glass film 5c with a negative (convex) curvature is formed, which can replace a nested inner element (also called "nested element" in the literature) like the inner ARE tube 5b.

Claims

1. 1. A method for making an anti-resonant hollow-core fiber having a hollow core extending along a longitudinal axis of the fiber and a cladding region including a plurality of anti-resonant elements and surrounding the hollow core, comprising: (a) providing a cladding tube (1) having an internal cladding bore (16) and a cladding longitudinal axis along which a cladding wall (2) defined by an inner and outer side extends; (b) providing a plurality of anti-resonant element preforms (5); (c) placing the anti-resonant element preform (5) at a target location inside the cladding wall (2) to form a primary preform (17) for the hollow-core fiber having a hollow core region and a cladding region; (d) drawing said primary preform (17) into said hollow-core fiber or further processing said primary preform (17) into a secondary preform (18) from which said hollow-core fiber is drawn; Equipped with In the further processing step, (i) stretching; (ii) collapse, (iii) collapsing and simultaneous stretching; (iv) collapse of additional cladding material; (v) collapsing of additional cladding material followed by stretching; (vi) collapsing and simultaneously stretching additional cladding material; In a manufacturing method, one or more of the following thermoforming processes are carried out once or repeatedly: The step of providing the cladding tube (1) according to step (a) comprises a machining step of providing a longitudinal structure (3) extending in the direction of the longitudinal axis of the cladding tube by machining the cladding tube wall (2) in the region of the target position, The method according to claim 1, characterized in that the longitudinal structure (3) has longitudinal slots distributed around the circumference of the cladding tube wall (2), and the anti-resonance element preforms (5) are arranged in each one of the longitudinal slots.

2. 2. The method according to claim 1, characterized in that the longitudinal structure (3) of the cladding tube wall (2) is produced by drilling, sawing, milling, cutting or grinding processes.

3. 2. The method according to claim 1, characterized in that a cladding tube (1) with a circular inner cross section is provided and the longitudinal structures (3) are configured as longitudinal slots.

4. 2. The method according to claim 1, characterized in that the cladding tube (1) has an end face end (12) and the longitudinal structure (3) terminates short of the end face end.

5. 2. The method according to claim 1, characterized in that when the anti-resonance element preforms (5) are placed at the target positions according to step (c), the anti-resonance element preforms each contact the longitudinal structure (3) at two edges (3a; 3b).

6. The longitudinal slot has parallel longitudinal edges (3a; 3b) and a maximum slot width S Ｂ The method according to any one of claims 1 to 5, characterized in that the anti-resonant element preform (5) is connected to the longitudinal edges (3a; 3b).

7. 7. A method according to claim 6, characterized in that the anti-resonant element preform (5) and the longitudinal edges (3a; 3b) are connected by softening with simultaneous drawing.

8. The method according to any one of claims 1 to 5, characterized in that the anti-resonant element preforms (5) each have at least one anti-resonant element (5a, 5b) and at least one capillary tube (9) connected to the anti-resonant element (5a, 5b), the capillary tube (9) being accommodated in a recess of the longitudinal structure (3), and when carrying out the process according to step (d), the capillary tube (9) is deformed by applying an internal pressure to the capillary tube inner bore.

9. Method according to any one of the preceding claims, characterized in that the anti-resonant elements (5) are arranged with odd symmetry around the hollow core.

10. 1. A method for making an anti-resonant hollow-core fiber preform having a hollow core extending along a longitudinal axis of the fiber and a cladding region including a plurality of anti-resonant elements and surrounding the hollow core, comprising: (a) providing a cladding tube (1) having an internal cladding bore and a cladding longitudinal axis, the cladding tube wall (2) being defined by an inner and outer side and extending along said cladding longitudinal axis; (b) providing a plurality of anti-resonant element preforms (5); (c) placing the anti-resonant element preform (5) at a target location inside the cladding wall (2) to form the hollow-core fiber primary preform (17) having a hollow core region and a cladding region; (d) optionally further processing said primary preform (17) into a secondary preform (18) for said hollow-core fiber; Equipped with In the further processing step, (i) stretching; (ii) collapse, (iii) collapsing and simultaneous stretching; (iv) collapse of additional cladding material; (v) collapsing of additional cladding material followed by stretching; (vi) collapsing and simultaneously stretching additional cladding material; In a manufacturing method, one or more of the following thermoforming processes are carried out once or repeatedly: The step of providing the cladding tube (1) according to step (a) comprises a machining step of providing a longitudinal structure (3) extending in the direction of the longitudinal axis of the cladding tube by machining the cladding tube wall (2) in the region of the target position, The method according to claim 1, characterized in that the longitudinal structure (3) has longitudinal slots distributed around the circumference of the cladding tube wall (2), and the anti-resonance element preforms (5) are arranged in each one of the longitudinal slots.

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